The patient took the drug for nearly 8 months before being diagnosed with the brain infection. The FDA issued an alert at the end of August to inform the public of its investigation.

The brain infection, sometimes fatal, is called PML (progressive multifocal leukoencephalopathy). The European case is the first reported in a patient who has not previously taken the drug Tysabri (natalizumab). Tysabri is already known to be linked with a higher risk for PML.

The maker of Gilenya, Novartis, issued a statement saying it had reviewed all available evidence and that the case of PML in Europe is unlikely to be linked to the drug.

Gilenya, approved by the FDA in 2010 for relapsing MS, is taken by mouth. It is one of three new oral drugs approved in the last 3 years. The other two are Tecfidera (dimethyl fumarate) and Aubagio (teriflunomide).

In MS, the immune system attacks the central nervous system, including the brain, spinal cord, and optic nerves.

According to the FDA, patients should not quit taking Gilenya without talking to their doctor.